It would be beneficial for different industries including those in the defense, law enforcement, environmental, food, medical, and materials fields to be able to detect trace amounts of gas-phase analytes using a reliable spectroscopic technique. Such a technique would allow them to detect contraband such as drugs, explosives, and/or contaminants on site. Unfortunately, however, very few spectroscopic techniques are sensitive enough to detect trace amounts of gas-phase analytes.
Intracavity laser absorption spectroscopy or “ICLAS” is one of the few spectroscopic techniques capable of doing so. In ICLAS, a test substance is introduced into the cavity of a laser that oscillates across a wavelength range. If the test sample contains a substance that absorbs in the wavelength range emitted by the laser, the absorption features affect the laser spectrum by a measurable amount. ICLAS is very sensitive because it allows for extremely long effective path lengths and high spectral resolution.
Many molecules have a characteristic vibrational and/or rotational absorption spectrum in a particular band of the infrared wavelength region. This band, which ranges from wavelengths of about 3 μm to about 12 μm, is known as the “molecular fingerprint region” because the fundamental rotational/vibrational absorption bands for most molecules fall within these wavelengths. Since each molecule exhibits a unique absorption spectrum in the fingerprint region, it is often used to qualitatively identify molecules.
Quantum cascade lasers or “QCLs” are promising laser sources for performing ICLAS in the infrared wavelength region because they have broad gain spectra, a wide range of wavelengths, high output power, high duty cycle, and the ability operate at room temperature. The fingerprint region is easily accessible with QCLs. Combining a QCL with the ICLAS technique allows one to obtain the highest possible absorption cross-section because of the long path lengths and wavelengths that may be employed.
Unfortunately, conventional intracavity laser absorption spectrometers are not without their drawbacks; namely, their sensitivity it is a function of how well all of their optical components are aligned.